This invention relates to a method of depositing metal on a surface and more particularly, to an improved method for the production of quality circuit boards with good line definition and clean non-conductor area as well as for extending the bath life of fixing solutions employed in the manufacture of printed circuit boards involving a photo-sensitive electroless deposition process.
The photosensitive process as originally developed by Polichette in U.S. Pat. Nos. 3,772,056, 3,722,078, 3,907,621, 3,930,963, 3,925,578, 3,959,547, 3,993,802 and 3,994,727, used water as the image developing media. The image was formed by a light sensitive mixture of a copper salt, anthraquinone disulfonic acid and a polyhydroxyl alcohol, such as glycerine, pentaerythritol or sorbitol. The mixture was applied from aqueous solution to a hydrophilic surface and subsequently dried under controlled conditions before light exposure. In the water development following light exposure the unexposed copper salts and anthraquinone disulfonic acid were rinsed away. The next step was reinforcement of the image formed by electroless plating to produce a copper conductor pattern.
In production, the process was plagued by copper deposits outside the conductor pattern. Increasing the water rinse or development either with longer time or more vigorous agitation resulted in a weakening of the image, characterized by breaks and voids in the conductor pattern. As an alternative to excessive rinsing or development, a number of intermediate process steps have been proposed for insertion between the water rinse or development and the electroless plating. The purpose of these steps is to fix the image before it is electrolessly plated.
Ehrich et al, U.S. Pat. No. 4,262,085, have proposed that the light image be intensified by immersion in a palladium sulfate solution followed by a nickel flash plating. Ehrich alleged that the undesired copper on the background from incomplete development would be removed and would not be a catalyst for electroless nickel. The nickel conductor can then be electroless copper plated to the required thickness without extraneous deposits.
Dafter, U.S. Pat. No. 4,084,023, has proposed a fixing solution consisting of an alkaline EDTA copper complexing solution and formaldehyde, a reducing agent for copper. The purpose of this solution was to dissolve away any incompletely developed copper from the background of the panel and at the same time prevent destruction of copper sites formed on the light image. This fixer solution was not completely successful.
Beckenbaugh, U.S. Pat. No. 4,181,750, proposed acid stripping solutions consisting of dilute citric or acetic acid solutions.
Madsen, U.S. Pat. No. 4,133,908, reported that the stripping and/or fixing solutions of Beckenbaugh, et al and Dafter were not effective. Madsen proposed an image stabilizer-potassium ferrocyanide addition to the Dafter fixing solution.
Beckenbaugh et al, U.S. Pat. No. 4,167,601 described the process of first fixing in Madsen's stabilizer solution followed by stripping in Beckenbaugh stripping solution.
Dafter, Beckenbaugh, Madsen and Beckenbaugh et al were all assigned to a common assignee.
Until now, the life span of fixing solutions employed in the above-described methods has been relatively short. Moreover, they were uncontrollable with respect to activity and adversely became colored after short periods of loading. As a result, workers in the field had to constantly replenish the fixing solutions, a measure which is time-consuming and costly.
It was believed that the aforedescribed shortcoming of fixing solutions was attributable to the build-up of certain contaminants after a short period of use. Simply replenishing the solutions by the addition of components did not solve the problem.
It is the objective of this invention to improve methods for making printed circuit boards involving fixing and electroless deposition steps by extending the bath life of the fixing solution employed. With the process improvement disclosed and claimed herein, the fixing solution is operable for extended periods of use and impurities which contaminate such solutions are effectively removed. The level of effectiveness of the so-treated solution is monitored spectrophotometrically, thus allowing for recycling of treated fixing solutions.